The result also provided Hammers boss Alan Pardew a satisfying victory against the club he walked out on to take over at the London outfit a year ago.
While the Hammers had only managed a goalless draw at Leicester City on the opening day of the Coca-Cola Championship campaign, Reading had got off to a fizzing start in their thrilling 3-2 win over Brighton.
And their second match equally looked set to yield goals aplenty, too, as both sides went on the offensive from the very first whistle.
Indeed, Andy Hughes wastefully scooped over inside 15 seconds while, at the other end, Marlon Harewood over-elaborated with just Royals keeper Marcus Hahnemann to beat, before strike partner Sheringham's glancing third-minute header drifted inches wide.
But although this end-to-end encounter may have been enthralling both the shirt-sleeved East End crowd and a live TV audience, it was rapidly becoming a defensive coach's nightmare.
Next up, Harewood scuffed wide after again finding himself eye-to-eye with the Reading keeper and Hayden Mullins twice missed by inches, before Dave Kitson, Nicky Forster and Steve Sidwell replied, forcing the alert Stephen Bywater into making crucial saves with close-range shots.
On 44 minutes, Harewood should have finally broken the deadlock but his point-blank volley cannoned back to him off Hahnemann and the rebound flew wide as the opening half somehow remained goalless.
Still the defensive deficiencies were only matched by the striking shortcomings in an equally cavalier second period.
The wasteful Harewood soon found himself firing far too close to the Reading keeper, before then ignoring the escaping Sergei Rebrov as he ghosted free.
Back in the West Ham penalty area, Hughes followed up his costly 15-second miss by slicing another close-range effort wide, while Kitson sent his point-blank 64th-minute effort straight into the relieved Bywater's clutches.
As the clocked ticked down, Bobby Zamora replaced the disgruntled, goal-shy Harewood while, more tellingly, Rebrov stepped down in favour of debutant Chadwick.
On 82 minutes, the former Manchester United starlet swept the ball back from the by-line for his ex-Old Trafford team-mate - making his 800th club appearance - to finally sweep home that priceless winner from six yards.